This section introduces aspects that may facilitate better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Mobile broadband will continue to drive the demand for a higher overall traffic capacity and a higher achievable end-user data rate in radio access networks. Several scenarios in the future will require data rates of up to 10 Gbps in local areas. Thus, an objective to be realized by next generation networks, which may be referred to as the fifth generation (5G) systems, is to meet such demand for the high traffic capacity and the high end-user date rate.
As one key technique, high-gain beamforming, typically implemented with array antennas, may be used in the next generation networks to mitigate the increased path loss at higher frequencies. Moreover, the next generation networks are also expected to be operating on the unlicensed (shared) spectrum in addition to the traditional licensed (exclusive) spectrum. It has been agreed to study Licensed-Assisted Access (LAA) technologies in the 3GPP at RP-141664. This LAA framework builds on carrier aggregation solutions introduced in LTE Release-10 to access the additional bandwidth in the unlicensed spectrum. Accordingly, a support for coexistence of the licensed and unlicensed spectrums is needed to enable spectrum sharing between different operators or systems. Currently, the Listen-Before-Talk (LBT) mechanism is the most flexible way to achieve this coexistence support, since it is a distributed mechanism such that there is no need for exchanging information between different operators or systems, which is costly and complicated.
The LBT mechanism is commonly used in Wi-Fi systems. Wi-Fi works with no physical wired connection between a sender and a receiver by using radio frequency (RF) technology. In Wi-Fi systems, an access point usually has a range of about 20 meters indoors and a greater range outdoors. Wi-Fi systems are defined as wireless local area network (WLAN) products based on Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards.
According to IEEE 802.11 standards, the basic media access control (MAC) protocol employs a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)-based LBT mechanism. FIG. 1 illustrates the LBT mechanism used in the current Wi-Fi systems. As illustrated, the same protocol is applied by all stations in both downlink (DL) and uplink (UL). A station, e.g. a user equipment (UE) that wishes to transmit a packet first senses the medium. If the medium is sensed idle for a certain time (the so-called Distributed Inter Frame Space (DIFS), e.g. 50 μs for 802.11b), the packet is then transmitted. If the medium is busy, the station defers until the medium is sensed idle. When this occurs, the station does not transmit immediately, since it may lead to collisions if more than one station was deferring. Instead, the station sets a backoff timer to a random value, and does not transmit until this timer has expired. The backoff timer is only decreased when the medium is sensed idle. Whenever the medium is sensed busy, a deferment state is entered where the backoff timer is not decreased. When the backoff timer expires, the packet will be transmitted. If the packet is successfully received by a receiving station, it responds with an acknowledgement to the transmitting station. The acknowledgement is sent a Short Inter Frame Space (SIFS), e.g. 10 μs for 802.11b, after the packet is received. Since SIFS<DIFS, no other station will access the medium during this time. If no acknowledgement is received by the transmitting station, because either the packet itself or the acknowledgement was lost, the transmitting station generates a new backoff, and retransmits the packet when the backoff timer has expired. Even if the packet was successfully acknowledged, the transmitter will generate a backoff and wait for it to expire before transmitting the next packet. The backoff time is measured in units of slot times, which for 802.11b are 20 μs long.
The current Wi-Fi systems are operating at low frequencies and thus both the “listen” and “talk” can be omni-directional. Therefore, the key objective of the LBT in Wi-Fi systems is to avoid interference between simultaneous data transmission. Practical application results show that the current LBT mechanism works well in Wi-Fi systems.
However, if the CSMA/CA based LBT is applied in the next generation networks at higher frequencies, due to directional transmission with beamforming, there will be some problems which do not exist in Wi-Fi systems. Therefore, a more efficient and effective LBT mechanism is needed for data transmission in the next generation networks with multiple beams and operable on the unlicensed spectrum, e.g. the LAA networks.